Transparent Exopolymer Particles in Deep Oceans: Synthesis and Future Challenges

Abstract

Transparent exopolymer particles (TEP) are a class of abundant gel-like particles that are omnipresent in seawater. While versatile roles of TEP in the regulation of carbon cycles have been studied extensively over the past three decades, investigators have only recently begun to find intriguing features of TEP distribution and processes in deep waters. The emergence of new research reflects the growing attention to ecological and biogeochemical processes in deep oceans, where large quantities of organic carbon are stored and processed. Here, we review recent research concerning the role of TEP in deep oceans. We discuss: (1) critical features in TEP distribution patterns, (2) TEP sources and sinks, and (3) contributions of TEP to the organic carbon inventory. We conclude that gaining a better understanding of TEP-mediated carbon cycling requires the effective application of gel theory and particle coagulation models for deep water settings. To achieve this goal, we need a better recognition and determination of the quantities, turnover, transport, chemical properties, and microbial processing of TEP.

Keywords: deep oceans; ocean carbon cycles; transparent exopolymer particles.

Figure 1

Vertical distributions of TEP concentrations, prokaryote abundance, and prokaryote production in the central Pacific Ocean. The graphs were made using the original data of Yamada et al. [19]. Both x and y axes are logarithmic.

Figure 2

Concentration ranges of DOC, high-molecular-weight DOC (HMW-DOC), POC, and TEP-C in the bathypelagic layer. Error bars represent the minimum and maximum values reported in the literature and the symbols indicate the midrange values. DOC and POC values are from Nagata et al. [13]. TEP-C is from Yamada et al. [19] and Engel et al. [25]. The large ranges for POC and TEP-C reflect both regional and seasonal variabilities. In addition, there are methodological uncertainties in the POC determination (e.g., [46]) and TEP-C estimation. Although analytical errors associated with colorimetric and microscopic methods of TEP estimation are generally small (standard deviations of replicated measurements are typically <10% of the mean values [8,15,19]), there are uncertainties in the conversion factor (or equation) relating TEP to carbon (see text). The range of DOC is smaller than the size of the symbol. HMW-DOC was assumed to be 20% of DOC [34].

Figure 3

Schematic representation of the major processes involved in the TEP dynamics in the deep ocean. (1) TEP are mainly produced by phytoplankton and bacteria in the upper ocean [5,6]. (2) A fraction of TEP produced in the upper ocean is transported to the deeper layers. The transport processes include the sinking of particles and the advective transport due to convection and mixing, and intrusion and lateral advection. TEP transport mediated by these processes is more important in the ocean’s margins than in open ocean domains. (3) TEP may be produced by the coagulation of nanogels, which are formed via the spontaneous assembly of DOC [2,3,4]. The ultimate source of DOC is primary production in the upper ocean, yet chemical characteristics of DOC and mechanisms underlying the persistence of DOC over millennia are poorly understood [1,38]. (4) Microbes may produce TEP, (5) while they also consume TEP. TEP are colonized by prokaryotes, acting as “hot spots” of microbes in deep oceans.